Underground reverse combustion process



Oct. 13, 1964 V T. M. GEFFEN ET AL 3,152,638

UNDERGROUND REVERSE COMBUSTION PROCESS Filed July 15, 1960 2 Sheets-Sheet 1 THEODORE M. GEFFEN HOWARD GREKEL KAROL L. HUJSAK INVENTORS ATTORNEY T. M. GEFFEN ET AL 3,152,638

UNDERGROUND REVERSE COMBUSTION PROCESS 2 Sheets-Sheet 2 Oct. 13, 1964 Filed July 15, 1960 THEODORE M. GEFFEN HOWARD GREKEL KAROL L. HUJSAK INVENTORJ ATTORNEY United States Patent 3,152,638 UNDERGROUND REVERSE COMBUSTIQN PROCESS Theodore M. Geffen, Howard Grekel, and Karol L.

Hujsak, Tulsa, Okla., assignors to Pan American Petroleum Corporation, Tulsa, ()lrlau, a corporation of Delaware Filed July 15, 1960, Ser. No. 43,089 18 Claims. (Cl. 166-2) The present invention relates to an improved method for conducting underground combustion of carbonaceous materials. More particularly, it is concerned with a novel method for increasing the area over which the combustion front travels during operations of this type.

In underground combustion of petroleum reservoirs, tar sands, coal, fractured shale, etc., particularly in the case of reverse combustion, there is evidence that the actual burned-out zone is narrow and is frequently referred to as a carrot or tear-shaped zone. In depth, this zone may extend the full dimension of the carbonaceous zone subjected to combustion. If it is desired to follow reverse combustion by a forward combustion step, the injected air tends to follow the same relatively narrow channel formed during the reverse burning phase. Accordingly, the recovery of valuable products can be so low as to be uneconomical.

It has previously been proposed that the sweep efliciency of the combustion front be improved by first performing a reverse burning step followed by forward combustion between two wells or two groups of wells. After this was accomplished, it was proposed that one or more offset wells could be drilled and used as injection wells thereby driving a wide combustion front toward the air injection area. However, in following reverse combustion by a forward burning step, it has been observed that the edges of the burned-out zone become plugged as the result of warm liquid products having been forced outwardly into colder zones during the forward burning step. This, of course, makes it exceedingly difficult, if not impossible, to establish air flow into the burned zone from the olfset injection well. Even if this could be done, it is most probable that the breakthrough would be uneven, traveling through the formation in streaks or fingers, and would not accomplish the purpose of forming a wide, even burning zone during the reverse combustion step.

Accordingly, it is an object of our invention to provide a method whereby a greater area is affected in a carbonaceous deposit in which an underground combustion process is carried out; in other words, we provide a means for spreading heat throughout a larger area of said deposit. It is a further object of our invention to employ this novel method of underground combustion in conjunction with other oil recovery techniques in order to obtain maximum oil production from a reservoir. It is another object of our invention to obtain a wider, more uniform burning front in underground combustion operations by providing conditions which produce an initial burning front corresponding in width to the distance between wells in which a reverse combustion step has just been carried out.

Reverse combustion of underground carbonaceous deposits is a technique that is now well known and has been described in Morse US. 2,793,696, as well as in other subsequently issued patents. Our invention, therefore, does not have as a basis for pateutability any direct relation to reverse underground combustion as now generally known. In accordance with a preferred form of the process of our invention, a reverse burning step is effected between two wells which penetrate a carbonaceous deposit such as, for example, tar sand. These wells may be 230 to 925 feet, or more, apart. Reverse combustion is carried 3,152,633 Patented Oct. 13, 1964 out between these wells so as to secure maximum product recovery. To secure this result, maximum temperatures of the order of about 700 to about 900 F., under the flux and pressure conditions required, are desirable. Further details of the conditions employed will be more specifically discussed below.

When an appreciable temperature rise, for example, 50 to about F. above normal formation temperature is reached at the face of the formation, in the injection well, air or other combustion-supporting gas is injected into an offset well or wells that may be, for example 230 to 925 feet away and positioned between the wells involved in the original reverse combustion step, but outside of the burned zone. The timing of air injection into the aforesaid offset well or wells is important. While we have indicated that such injection should preferably occur at the peak temperature at the face of the original injection well, the procedure may be modified so that air injection is begun either just before or just after the maximum temperature of said face is reached. This condition can be readily ascertained after one has become familiar with the temperatures produced in a given carbonaceous deposit at a given air flux and pressure. We desire to begin air injection in the above-mentioned offset Well under the temperature conditions just described because if the original combustion process is permitted to reverse itself with the combustion front traveling in the same direction as the gas flow, hot viscous products will contact the outer cooler perimeter of the burned-out zone and plug or seal the entire envelope defining said zone. In other words, air injection into the offset well should occur prior to substantial flow of tar or oil toward the outer edges of the burned area.

As a matter of explanation as to why this plugging occurs during forward burning, it should be pointed out that in such operation the area ahead of the combustion zone is the coolest. With the air flow being through the hot zone and into the cooler zone, the hot mobile liquid products tend to fan out into the formation ahead of the burning front into the cooler portion of the formation ahead of said front. On contact of said products with the cooler formation, they naturally become more viscous and ultimately will form a seal in the area where the formation is not hot enough to keep their viscosities at a sufliciently low value.

By injecting air via an offset Well system we are able to conduct a second reverse combustion step in which the width of the front in the latter operation corresponds approximately to the distance between the production and injection wells involved in the first reverse burning step. Inthis manner it is possible to extend the burning operation to a greater area with a minimum of wells.

In carrying out one embodiment of the process of our invention, the peak reverse combustion temperature may range from about 600 to 1000 F., and preferably from about 700 to about 900 F. In the case of Athabasca tar sand, very little, if any, of the tar vaporizes at temperatures below 400 F. Distillation increases with rising temperature, and cracking becomes appreciable above about 500 F. and increases with increasing temperature. As temperatures increase, more of the oil or tar is burned. Distillation and cracking are essentially complete at a temperature of about 900 F. Accordingly, above temperatures of about 900 F., the recovery declines significantly because of the additional amount of hydrocarbon necessary for the combustion step. A factor affecting or determining combustion zone temperature is, of course, air injection rates or air flux, referred to as s.c.f. of air/ hr./ft. Generally speaking. fluxes of the order of 50 to 175 s.c.f./hr./ft. and preferably 100 to about s.c.f./ hr./ft. should be used at the pressures normally involved to produce reservoir temperatures of the order of 700 to 900 F.

Another factor affecting the temperature of the combustion zone is the pressure prevailing in that zone during combustion. This is readily shown when the system is operated at constant air injection rates. For example, in a series of tests carried out in Athabasca tar sands, combustion zone temperatures of about 880 F. were generated at air injection rates of the order of 50 s.c.f./hr./ ft. and at pressures near atmospheric, i.e., about to 25 p.s.i.a. When the pressure was increased to 265 p.s.i.a., the combustion zone temperature was found to decrease to about 550 F. at the same air rate. Likewise, at an air injection rate of 50 s.c.f./hr./ft. the combustion zone velocity at pressures of the order of 15 to 25 p.s.i.a. was about 0.14 ft./hr.; whereas, at 265 p.s.i.a., the velocity was about 0.29 ft./hr. at the same air rate. Thus, it will be seen that both combustion zone velocity and temperature are dependent upon reservoir pressure.

The pressures used in carrying out the process of the present invention may vary Widely, depending upon a number of conditions such as the permeability of the reservoir, the desired combustion zone temperature, the air injection rate, etc. Ingeneral, it may be said that the pressure employed should exceed that of the reservoir and, in some instances, it may be desirable to employ pressures sufficiently high to fracture the formation in order to achieve the desired air flux and temperature. Typically, in the case of tar sand operations, we desire to use pressures of the order of 200 to 1000 p.s.i., and preferably from about 200 to 500 p.s.i. Generally, at these pressures, i.e., 200 to 500 p.s.i., air fluxes of the order of 100 to 150 s.c.f./hr./ft. respectively, should be used in order to operate with a temperature range of from about 700 to 900 F.

The process of our invention may be further illustrated by reference to the accompanying drawings, in which FIGURES 1 and 2 show the basic concept, while FIG- URE 3 is a diagrammatical representation of a special application of this concept.

Referring, now, to FIGURES 1 and 2, original producing well 2 and original injection Well 4 are joined by a burned-out area 6 formed as the result of a reverse combustion process. Air injection into well 4 is discontinued and switched to well 8 when the temperature at the face of well 4 reaches a level of about 100 F. higher than the natural temperature of the carbonaceous deposit, or at least has not decreased in temperature more than 50 F. from the maximum. This, of course, may include situations where the formation face has reached a temperature in excess of the aforesaid 100 F., but is in a cooling cycle in which the temperature of said face is only about 50 F. higher than the natural temperature of the deposit.

The distance between these wells may be of the order of 165 to 660 feet. If a suitable air flux cannot be obtained to maintain a temperature of the order of about 700 to 750 F., the formation between the wells may be fractured in accordance with known methods.

Continued injection of air through well 8 at about 400 p.s.i. and at a flux of about 125 s.c.f./hr./ft. results in the production of an oily product from both wells 2 and 4, having an A.P.I. gravity of about and a viscosity of about 9 cp. As previously pointed out, combustion zone temperatures in the range of from about 700 to about 900 F. are preferable because the product obtained, particularly with tar sands, has been cracked to a substantial extent and, hence, contains an increased fraction of more valuable hydrocarbons. However, we have also observed that liquid products can be obtained by operating at temperatures of above 400 but below about 500 F. Such products, in gravity and other characteristics, resemble very closely those of the tar in its natural state.

FIGURE 3 embodies a multiple well system in which advantage may be taken of the improved sweep efficiency provided by our invention. Thus, as in FIGURES 1 and 2, the perimeter wells may be from about to about 660 feet apart. In a given pair of perimeter wells, the formation in well 14 is ignited and air injected via well 12. Reverse combustion occurs and defines a hot zone 16 as the result of the burning front traveling from wells 14 to 12. When the temperature in the injection wells 12 approaches or reaches the maximum, injection of air in these wells is discontinued and they, along with wells 12, are converted to producing wells by injecting air into one or more of wells 18. Under these conditions, reverse combustion occurs in which burning fronts defined by zones 16 move toward and converge on injection wells 18. It will be seen that in this manner a burning front is given ample opportunity to contact the unburned portion enclosed by hot zones 16 and surrounding well 13. In practice, this procedure may be started as a conventional five-spot system (as indicated in the square formed by dashed lines), and as the burning progresses, additional wells may be drilled, in accordance with the pattern shown, with a single injection well 18 ultimately serving a total of 4 producing wells 14 and 12. Other ramifications of the above system will, of course, be apparent to those skilled in the art without departing from the scope of our invention.

Also, it is to be pointed out that in a sizeable operation such as contemplated for a commercial application, various secondary recovery operations may be applied to the particular area of the project in which burning is no longer actively occurring, i.e., after air injection in a well 18 has been discontinued. Thus, as one side of the area is burned, or worked out, any remaining recoverable oil in the worked-out area can be subjected, for example, to forward combustion. Our invention likewise contemplates conducting reverse combustion procedures under conditions such that no production is obtained during either or both of the combustion steps. For example, it may be that the reservoir under consideration has such low permeability that the possible air flux can generate a reservoir temperature of not more than 300 or 400 F. If temperatures no greater than this can be achieved, or if for some reason it is desirable to control the air injection rate so as not to exceed such temperatures, it has been our experience that no significant production, particularly in the case of tar sands, will flow to the producing wells. By conducting reverse burning operations so that the maximum temperatures reached are of the order of 300 to 400 F., the tar or other carbonaceous material become less viscous, rendering the reservoir more permeable and, therefore, more amenable to other methods of oil recovery such as, for example, forward combustion, waterfiooding, etc.

The term carbonaceous, as used herein, is intended to refer to materials comprising either free or combined carbon.

We claim:

1. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises effecting a reverse burning operation in said deposit between a producing well and an injection well, both of said wells penetrating said deposit, recovering fluid products from said producing well, continuing said reverse burning operation until the face of the formation of said injection well has reached a temperature in excess of the normal temperature of said deposit, next discontinuing said reverse burning operation, thereafter initiating injection of a combustion-supporting gas into said deposit while the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature, said last-mentioned injection step being via an offset well located at a point which, when projected, falls between said injection and producing wells and on a line connecting said producing and injection wells whereby a second reverse combustion operation is effected having a burning front corresponding in length essentially equal to the distance between said producing and injection wells, said front moving toward said oifset well, and recovering fluid products from said injection and producing wells, said offset well being the only one of the three wells involved in which said gas is injected after said reverse burning operation has been discontinued.

2. The process of claim 1 in which said carbonaceous deposit is a tar sand.

3. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises eifecting a reverse burning operation in said deposit between a producing Well and an injection well, both of said wells penetrating said deposit, recovering fluid products from said producing well, continuing said reverse burning operation until the face of the formation of said injection well has reached a temperature in excess of the normal temperature of said deposit but not more than about 100 F. higher than said normal temperature, discontinuing said reverse burning operation, initiating injection of a combustion-supporting gas into said deposit while the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature, said last-mentioned injection step being via an offset well located at a point which, when projected, falls substantially midway between said injection and producing wells perpendicular to a line connecting said producing and injection wells, said gas being injected at a pressure of from about 200 to about 1000 p.s.i., and recovering fluid products from said producing and injection wells, said offset well being the only one of the three wells involved in which said gas is injected after said reverse burning operation has been discontinued.

4. The process of claim 3 in which the carbonaceous deposit is tar sand.

5 The process of claim 3 in which the pressure ranges from about 200 to about 500 p.s.i.

6. The process of claim 5 in which said deposit is tar sand.

7. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises eifecting a reverse burning operation in said deposit between a producing well and an injection well by igniting said deposit in said producing well, introducing a combustion-supporting gas into said deposit via said injecton well at a pressure ranging from about 200 to about 1000 p.s.i., recovering fluid products from said producing well, continuing said reverse burning operation until the face of the formation of said injection well has reached a temperature in excess of the normal temperature of said deposit but not more than about 100 F. higher than said normal temperature, next discontinuing said reverse burning operation, initiating injection of said gas into said deposit while the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature, said last-mentioned injection step being via an offset well located at a point which, when projected, falls substantially midway between and perpendicular to a line connecting said injection and producing wells, said gas beng injected at a pressure of from about 200 to about 1000 p.s.i., and recovering fluid products from said producing and injection well, said offset well being the only one of the three wells involved in which said gas is injected after said reverse burning operation has been discontinued.

8. The process of claim 7 in which the pressure employed in both reverse burning steps ranges from about 200 to about 500 p.s.i.

9. The process of claim 8 in which said carbonaceous deposit is tar sand.

10. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises the use of a pair of diagonally opposed injection Wells and a pair of diagonally opposed producing wells, said producing and injection wells being in a given rectangular area, conducting a reverse burning operation be tween adjacent producing and injection wells by injecting via the latter wells a combustion-supporting gas into said deposit at a pressure of from about 200 to about 1000 p.s.i. until a temperature at the face of said injection wells in excess of the normal temperature of said deposit but not more than about F. higher than said normal temperature has been reached, discontinuing said reverse burning operation, thereafter injecting said gas into said deposit while the burned zone resulting from the aforesaid reverse burning operation as at combustion-supporting temperature via still anothmer injection well located substantially centrally of the area defined by said first-mentioned producing and injection wells at a pressure within the respective ranges set forth above, and recovering fluid products from said first-mentioned producing and injection wells.

11. The process of claim 10 in which said combustionsupporting gas flux and the pressure employed in both reverse burning steps range from about 100 to s.c.f./ hr./ft. and from about 200 to about 500 p.s.i., respectively.

12. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises the use of a pair of diagonally opposed injection wells and a pair of diagonally opposed producing wells, said producing and injection wells being in a given rectangular area, conducting a reverse burning operation between adjacent producing and injection wells by injecting a free oxygen-containing gas into said deposit at a pressure of from about 200 to about 1000 p.s.i. until a temperature of from about 50 to about 100 F. higher than the normal temperature of said deposit has been reached, discontinuing said reverse burning operation, thereafter injecting said gas into said deposit While the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature via still another injection Well located substantially centrally of the area defined by said first-mentioned producing and injec tion wells at a flux and at a pressure within the respective ranges set forth above, recovering fluid products from said first-mentioned producing and injection wells, continuing introduction of said gas under the aforesaid conditions via said still another injection well until the temperature of the face of the formation of said still another injection well has reached a temperature of from about 50 to about 100 F. higher than the normal temperature of said deposit, and recovering fluid products from said firstmentioned producing and injection wells.

13. The process of claim 12 in which said deposit is subjected to a forward combustion operation after the second reverse combustion step has been completed.

14. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises eifecting a reverse burning operation in said deposit between a producing well and an injection well, both of said wells penetrating said deposit, continuing said reverse burning operation until the face of the formation of said injection well has reached a temperature in excess of the normal temperature of said deposit but not more than about 100 F. higher than said normal temperature, next discontinuing said reverse burning operation, thereafter initiating injection of a combustionsupporting gas into said deposit while the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature via an offset well located at a point which, when projected, falls substantially midway between and perpendicular to a line connecting said injection and producing wells whereby a second reverse burning operation is eifected having a burning front corresponding in length essentially equal to the distance between said producing and injection wells, said front moving toward said olfset well, said offset well being the only one of the three wells involved in which said gas is injected after said reverse burning operation 1? has been discontinued, and recovering fluid products from said first-mentioned producing and injection wells.

15. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises efiecting a reverse burning operation in said deposit between a producing well and an injection well, both of said wells penetrating said deposit, continuing said reverse burning operation until the face of the formation of said injection well has reached a temperature in excess of the normal temperature of said deposit but not more than about 100 F. higher than said normal temperature, discontinuing said reverse burning operation, and thereafter initiating injection of said gas into said deposit While the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature, said last-mentioned injection step being via an offset well located at a point which, when projected, falls between said injection and producing wells perpendicular to a line connectin said producing and injection wells, said gas being injected at a pressure of from about 200 to about 1000 p.s.i., said oflset well being the only one of the three wells involved in which said gas is injected after said re verse burning operation has been discontinued, and recovering fluid products from said first-mentioned pro ducing and injection wells.

16. In a process for effecting underground combustion in a carbonaceous deposit, the improvement which comprises the use of a pair of diagonally opposed injection wells and a pair of diagonally opposed producing wells,

said producing and injection wells being in a given rect2; tangular area, conducting a reverse burning operation between adjacent producing and injection wells by injecting via the latter wells a combustion-supporting gas into said deposit at a pressure of from about 200 to about 1000 p.s.i. until a temperature at the face of said injection wells in excess of the normal temperature of said deposit but not more than about F. higher than said normal temperature has been reached, discontinuing said reverse burning operation, and thereafter injecting said gas into said deposit while the burned zone resulting from the aforesaid reverse burning operation is at combustion-supporting temperature via still another injection well located substantially centrally of the area defined by said firstmentioned producing and injection wells at a flux and at a pressure within the respective ranges set forth above, and recovering fluid products from said first-mentioned producing and injection wells.

17. The process of claim 15 in which said deposit is subjected to a forward burning operation after the second reverse burning step has been completed.

18. The process of claim 16 in which said deposit is subjected to a forward burning operation after the second reverse burning step has been completed.

References Cited in the file of this patent UNITED STATES PATENTS 2,888,987 Parker June 2, 1959 2,899,186 Crawford Aug. 11, 1959 2,994,376 Crawford et al Aug. 1, 1961 3,057,403 Wyllie Oct. 9, 1962 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3, 152,638 Oe-t-ober- 13, 1964 Theodore M. Geffen -et---al. I It is hereby certified thaterror appears in the above numbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column 3, line 48, after "than" insert about 1---; column 6, line 11, for "as" read is 1ine- I2,--for "'anothmer" read another Signed and sealed this 16th day of February 1965.

(SEAL) Attest:

ERNEST W. SWI'DER EDWARD.- J. BRENNER Attesting Officer Commissioner of Patents 

1. IN A PROCESS FOR EFFECTING UNDERGROUND COMBUSTION IN A CARBONACEOUS DEPOSIT, THE IMPROVEMENT WHICH COMPRISES EFFECTING A REVERSE BURNING OPERATION IN SAID DEPOSIT BETWEEN A PRODUCING WELL AND AN INJECTION WELL, BOTH OF SAID WELLS PENETRATING SAID DEPOSIT, RECOVERING FLUID PRODUCTS FROM SAID PRODUCING WELL, CONTINUING SAID REVERSE BURNING OPERATION UNTIL THE FACE OF THE FORMATION OF SAID INJECTION WELL HAS REACHED A TEMPERATURE IN EXCESS OF THE NORMAL TEMPERATURE OF SAID DEPOSIT, NEXT DISCONTINUING SAID REVERSE BURNING OPERATION, THEREAFTER INITIATING INJECTION OF A COMBUSTION-SUPPORTING GAS INTO SAID DEPOSIT WHILE THE BURNED ZONE RESULTING FROM THE AFORESAID REVERSE BURNING OPERATION IS AT COMBUSTION-SUPPORTING TEMPERATURE, SAID LAST-MENTIONED INJECTION STEP BEING VIA AN OFFSET WELL LOCATED AT A POINT WHICH, WHEN PROJECTED, FALLS BETWEEN SAID INJECTION AND PRODUCING WELLS AND ON A LINE CONNECTING SAID PRODUCING AND INJECTION WELLS WHEREBY A SECOND REVERSE COMBUSTION OPERATION IS EFFECTED HAVING A BURNING FRONT CORRESPONDING IN LENGTH ESSENTIALLY EQUAL TO THE DISTANCE BETWEEN SAID PRODUCING AND INJECTION WELLS, SAID OFFSET WELL BEING THE ONLY ONE OF THE THREE WELLS INVOLVED IN WHICH SAID GAS IS INJECTED AFTER SAID REVERSE BURNING OPERATION HAS BEEN DISCONTINUED. 